FIG. 1 shows a typical power steering apparatus 10 for use in automobiles to which an automatic steering center setting apparatus of the present invention can be applied.
As shown in the diagram, a vehicle velocity signal from a vehicle velocity sensor 1 and a steering angle signal from a steering angle sensor 2 adapted to detect a rotational deviation of a steering wheel 11 are supplied to an electronic controller 12 such as a microcomputer or the like. On the basis of a preset program, the electronic controller 12 outputs a steering assist signal in response to the vehicle velocity indicated by the vehicle velocity signal, and the steering direction and the steering angle indicated by the steering angle signal. A hydraulic apparatus 13 is made operative by the steering assist signal and supplies a hydraulic pressure to a hydraulic pressure control valve 15 which is interlocked with a steering mechanism 14 such as a rack and pinion apparatus. Thus, power assistance suitable to the running state of the automobile and to the steering angle of the steering wheel 11 can be accomplished. In other words, the power steering apparatus as shown in the diagram has a vehicle velocity responsive characteristic and a steering angle responsive characteristic.
The steering assist signal may not be used to make the hydraulic apparatus 13 operative but may instead be directly supplied to an electric motor to actuate the steering mechanism.
In such a power steering apparatus, particularly in a power steering apparatus having a steering angle responsive characteristic, in order to adjust the steering force in accordance with the right and left steering angles, the steering center needs to be detected or determined by a steering angle sensor.
In the conventional power steering apparatus having a steering angle responsive characteristic, the function of the steering angle responsive characteristic is started simultaneously with the start of a steering operation. Therefore, when the position of the steering center, which is detected by the angle sensor and serves as a reference for the steering power assistance responsive to the steering angle, deviates from the true steering center position the steering angle responsive characteristic suddenly functions and the steering power assistance suddenly changes. Therefore, the steering stability is adversely affected and this may cause an accident.
On the other hand, a potentiometer is used as a conventional angle sensor adapted to detect the steering center position in order to detect any change in electric resistance value depending on the steering operation, or alternatively a mechanical sensor adapted to detect a movement deviation of a rack bar in a steering mechanism having a rack and pinion structure is employed. However, the life of such sensors is short and they are expensive to provide. In particular, the mechanical sensor has a drawback in that the structure is complicated since it is attached to the steering mechanism.